1Field of the Invention
The present invention relates to high speed communications systems for digital data; and, more particularly, provides a data communications systems relying on analog signals for transmission of digital data.
2. Description of Related Art
High speed communications are required for a growing number of applications. The amount of data that can be transmitted per unit time has been limited by the switching speed of components in the transmitters and receivers. Further, for data integrity it has been the practice to use digital communication signals that include one bit of information per cycle.
It is known that a multi-level signal, such as the quasi-analog pulse amplitude modulation scheme (PAM) shown in FIG. 1a, contains more information per pulse than a pure digital signal. More information per pulse means more information per cycle in the transmitted signal and a higher effective data rate at a given frequency. Alternative quasi-analog systems shown in FIGS. 1b and 1c, respectively, are pulse width modulation and pulse position modulation. These quasi-analog communication signal formats have been used for the purposes of sampling analog data, such as voice in a telephone system, and converting it to a format for communication.
For long distance communications, these three quasi-analog communication signals have been converted to a purely digital format, such as pulse code modulation, to overcome noise interference problems and the like. Once in a purely digital format, the data can be combined with other kinds of digital data for high speed - long distance communication.
The limitation in switching speed of integrated circuit components begins to limit the speed of transmission of data in a digital format in current high volume data transmission systems. Such rapid communication applications are growing and include, in addition to telecommunications, video teleconferencing, digital video image transmission, and the like. Thus, it is desirable to increase the effective data rate of a transmission system having a given switching speed limitation.